1. FIELD OF THE INVENTION:
The invention relates to downhole safety valves for subteranean wells and particularly to a method and apparatus for effecting the replacement of a defective full bore opening safety valve by a secondary safety valve which is sealably inserted in the bore opening of the original safety valve and operated by control fluid supplied through the existing control fluid piping.
2. SUMMARY OF THE PRIOR ART:
A very popular form of downhole well safety valve comprises the so-called "full bore opening" type which refers to a safety valve wherein the bore opening through the valve, when it is disposed in its open position, is substantially equal to the internal bore diameter of the tubing string in which the safety valve is incorporated. Such full bore opening valves may employ a rotatable ball,or a pivoted flapper, as the shiftable valve head. In either case, the valve head is shifted to its full open position by an actuating sleeve which is axially shiftably mounted within the bore of the valve housing and is operated by one or more hydraulic cylinders to shift the actuating sleeve downwardly and effect the movement of the valve head to its full open position. Pressured control fluid for operating the cylinder is supplied from the surface by small diameter pipe or tubing which communicates with a control fluid passage in the wall of the valve housing. Of course, in such full open position, the actuating sleeve is spring biased to its valve closing position.
A valve of this general type is shown in U.S. Pat. Nos. 4,503,913 and 4,796,705. The latter patent provides a secondary actuating cylinder for effecting the locking of the movable head of the safety valve in its full open position in the event of any failure or defect in the operation of the primary cylinder.
It is, of course, necessary to replace the defective safety valve by a functional safety valve and this has been accomplished in the past by inserting so-called in-tubing safety valves within the bore of the original defective safety valve while the original defective valve is in its locked, full open position.
Such replacement valves are generally inserted by wireline, hence the problem arises as to how the already installed control fluid piping can be utilized to effect the control of the replacement valve.
U.S. Pat. No. 3,696,868 discloses an in-tubing replacement valve for an installed defective safety valve wherein the wall of the actuating sleeve for the defective valve is perforated prior to insertion of the replacement valve to provide communication with the existing control fluid conduits. Obviously, the production of perforations in an installed sleeve without damaging surrounding elements is a difficult operation.
Prior art arrangements have also utilized ports in the original valve housing communicating with the control fluid conduit and provided seal elements for such ports or threaded plugs which are removed by the insertion of the replacement safety valve. Obviously, any time that a seal or a threaded plug is employed in a downhole environment, there is the distinct possibility that such seal will leak and produce undesirable effects on an operation of the well.
U.S. Pat. #3,799,258 proposes the utilization of a hollow shearable threaded nipple traversing the wall of the valve housing with the inwardly projecting end of the nipple being sheared off by a sleeve which is moved downwardly by "a suitable tool". This arrangement has several obvious disadvantages.
Since the original safety valve may function properly for many months, the successive passages of well treatment and/or measuring tools dowardly through the safety valve always involves the danger that the inwardly projecting end of the hollow nipple may be accidentally sheared off, thus rendering the installed safety valve inoperative. Furthermore, leakage around the threaded nipple by high pressure, highly corrosive well fluids is a constant threat.
Accordingly, the prior art has not provided an adequate solution to the problem of effecting trouble free fluid communication of control fluid to a replacement in-tubing safety valve through the piping and fluid passages already existing in an installed defective safety valve.